1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In the field of electrophotographic image recording, image forming apparatuses that use lasers as light sources are widely used. Such image forming apparatuses include optical scanning devices that emit light from a light source, deflect the light with a deflector, scan the surface of a drum-shaped photosensitive element with the deflected light (scanning light), and form a latent image on the surface of the photosensitive element.
Multi-color image forming apparatuses have been developed that form an image by superimposing different color images on each other. Tandem-type multi-color image forming apparatuses are particularly widely used as examples of such apparatuses. They form, using a plurality of scanning optical systems, light spots on the surfaces of a plurality of color-based drum-shaped photosensitive elements.
One technology that copes with multi-color image writing uses a plurality of optical scanning devices so that each optical scanning device in the plurality is for a different color; however, using this technology increases the number of necessary parts and makes it difficult to reduce the size of the image forming apparatus.
One technology that solves the above problems uses one optical scanning device that includes a plurality of optical scanning systems. Such an optical scanning device, in general, includes one deflector that receives and deflects a plurality of beams of light of different colors. Each deflected beam of light passes through a corresponding optical scanning system and forms an image on the surface of a corresponding photosensitive element. However, because several sets of optical elements for different colors are arranged near the deflector, it is still difficult to produce an optical scanning device that is sufficiently small.
One technology that can be used to produce an optical scanning device that is sufficiently small uses an optical scanning system that includes a polarized light splitter (polarized light separator) and splits light depending on the light's polarization direction (see, for example, Japanese Patent No. 3247497, Japanese Patent Application Laid-open No. 2008-070599, Japanese Patent Application Laid-open No. H7-144434, and Japanese Patent Application Laid-open No. H10-3048). Using this technology is an effective way to decrease the thickness of an optical scanning device, i.e., the length of the optical scanning device in the rotation axial direction of the deflector. Using this technology is also an effective way to reduce the number of necessary parts because some optical elements within the scanning optical system are shared.
Another technology that is used to produce an optical scanning device that is sufficiently small uses a dynamic active element, such as a spatial modulator. Another technology uses a dichroic mirror and splits light according to various wavelengths. However, the former technology needs an additional circuit to drive the active element, and the latter technology needs different types of light sources in one optical scanning device. Using these technologies is an effective way to reduce the number of optical elements in the scanning optical system; however, the downside is that an expensive element is needed. Therefore, technology that uses a passive element or a polarized light splitter (hereinafter, “polarized-light splitting technology”) is preferable.
To produce a slim optical scanning device using the polarized-light splitting technology, an excellent polarized-light splitting characteristic is needed. If a polarized light splitter receives, for example, slightly elliptically polarized light or polarized light with the polarization direction slightly different from the expected polarization direction, the light is not split correctly and part of the light to be output to a first scanning optical system is output to a second scanning optical system.
The beams of light to be split are modulated in accordance with different time-series signals so that image data is written to each surface with the corresponding beam of light. If the polarized-light splitting characteristic is not adequate, part of the image data for the different surfaces to be scanned becomes mixed. For example, when this happens in a multi-color image forming apparatus, the part of the data to be developed with cyan is written to the surface for magenta, which results in an image with color crosstalk.
The degradation of the polarized-light splitting characteristic occurs due to, for example, birefringence peculiar to resin scanning lenses. Although studies on low-birefringence resin materials have been widely conducted, the use of low-birefringence resin scanning-lenses is still difficult in practice due to difficulties in shaping, molding, and efficiently manufacturing the lenses.
One easy solution for avoiding the birefringence phenomenon is to use glass for all the lenses in the scanning optical system. However, to satisfy the current needs for high image quality, it is required to increase the number of lenses and the accuracy of the lens processing, which increase costs.
The problem is that, in the technologies disclosed in Japanese Patent No. 3247497, Japanese Patent Application Laid-open No. 2008-070599, Japanese Patent Application Laid-open No. H7-144434, and Japanese Patent Application Laid-open No. H10-3048, the degradation of the polarized-light splitting characteristics caused by the birefringence peculiar to resin scanning lenses is not taken into consideration.